Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/10—Code generation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
  • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
  • H04B7/2628—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using code-division multiple access [CDMA] or spread spectrum multiple access [SSMA]
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/0022—PN, e.g. Kronecker
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/004—Orthogonal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/16—Code allocation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/70703—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation using multiple or variable rates

Definitions

• This invention is generally related to telecommunications systems and apparatus that employ spreading codes and, in particular, relates to methods and apparatus for generating a set of spreading codes that are optimized for a multiuser, multi-rate environment.
• CDMA Code Division, Multiple Access
• PN pseudonoise
• the forward channel of most CDMA systems utilizes some form of synchronous CDMA.
• the reverse channel i.e., subscriber unit to base station
• a timing control loop is utilized to maintain the various users in the system time-aligned such that their respective signals all arrive at the base station within a small fraction of a chip of each other.
• PN codes which are designed to have the smallest possible cross-correlation when time- aligned with each other. If the number of users in the system is less than the number of chips transmitted for each channel symbol (which may be referred to as the channel symbol processing gain) , then it is possible to design PN codes that are truly orthogonal to each other. When the number of users exceeds the channel symbol processing gain, then it is no longer possible to design codes that are orthogonal, since the dimensionality of the signaling space has been exceeded.
• the system It is often desirable for the system to support users which are not all at the same signaling rate. For example, in a system where some users are using a telephone and the required date rate is on the order of a few thousand bits per second (Kbps) to a few tens of Kbps, while other users are using the system as a computer network interface and require a million bits per second (Mbps) or more, the waveform should to be able to simultaneously accommodate the various non-homogeneous users.
• Kbps bits per second
• Mbps million bits per second
• a more cost-effective technique to support high rate and low rate users simultaneously is to employ a common chipping rate for all users, but to permit the users in the system to vary their channel symbol processing gains depending on their respective data rate. This implies that if one desires that all users in either the forward, or the forward and reverse channels, to be orthogonal to each other, independent of their rates, then a set of PN codes are needed of various lengths, and that are mutually orthogonal when synchronized appropriately.
• Walsh functions are a set of binary and orthogonal waveforms that can be used for signal multiplexing purposes, and have long been recognized as having application to telephony. Reference in this regard can be had to an article entitled "The Multiplexing of Telephone Signals by Walsh Functions", by I.A. Davidson in Applications of Walsh Functions, 1971 proceedings, Second Edition, Eds. R.W. Zeek and A.E. Showalter, pages 177-179.
• Walsh functions are special Hadamard functions, and can be described by Hadamard matrices with powers of 2 as ordinary numbers. Further function systems can be derived from Hadamard matrices by permutation of columns and rows and by sign inversion, while preserving their orthogonal characteristics.
• w.. (n) be the j th ⁇ 1 valued chip in the i th row of the code matrix. If only one subscript is used, w ⁇ n), let that represent the i th row of the code matrix, or in other words, the i th PN code in the set having n chips in the vector.
• both i and j are integers ranging from 1 to n.
• Fig. la it can be seen that there is a choice of supporting n users at rate Rc/n, 2n users at rate Rc/2n, or a number of users between n and 2n at mixed rates. If, for example, a user employs code w,(n) at rate Rc/n, then codes w 1 (2n) and w 2 (2n) at rate Rc/2n may not be used, since they are not orthogonal with the user employing code w ⁇ n) at rate Rc/n. Other users may employ codes w 3 (2n) and w 4 (2n) at rate Rc/2n since they are all mutually orthogonal, even though they are at different data rates.
• a problem that arises when applying a cover code to the matrix is that the resulting randomized Walsh codes are not balanced. This means that, over any symbol period, the number of +1 valued chips and -1 valued chips are not equal to one another in most of the resulting PN codes.
• Balance in the code set is a very desirable property, since it implies that the codes are orthogonal to any DC offset in the receiver of the signal. In other words, if the chips are ⁇ 1 millivolts in the receiver, but there is a 2 millivolt DC offset in the signal at the input of the despreader, then the despreader would have to multiply the ⁇ 1 despreading code with an input signal having values of +3 and +1 millivolts. However, if the PN code is balanced over a symbol, then the DC offset will not affect the despreading process.
• a method for constructing a series of mutually orthogonal sets of PN codes which support multirate signaling is disclosed.
• This series of sets of PN codes has the desirable properties that the constituent codes are balanced at all of the desired symbol rates, and also exhibit good spectral properties (provide a data randomization function) .
• this series of sets of PN codes permits efficient multicell operation, since any constituent code of one set appears to be approximately random relative to any constituent code of any other set.
• These improved codes are can be used to advantage in the forward channel (point-to-multipoint direction) of CDMA systems, but may also be employed in the reverse channel if the reverse link employs quasi-synchronous CDMA.
• a non-recursive technique for constructing a series of PN code sets that can be used for multirate synchronous and quasi-synchronous CDMA systems.
• the construction technique is superior to conventional techniques in that it produces PN codes that are balanced, and that furthermore do not require any synchronization of neighboring base stations.
• This approach may be characterized as using a permuted orthogonal matrix to modulate permuted orthogonal matrices to create PN codes that support multirate operation.
• the codes constructed using the method of this invention have very good spectral properties (if chosen properly) when the code length, n, is reasonably large.
• a non-recursive method for constructing balanced PN code sets for use in a CDMA communication system.
• the method includes steps of (a) applying a constrained permutation to a GxG +1 valued matrix to form a modulation matrix M(G) ; and (b) using the modulation matrix M(G) to create a set of available PN codes for a first cell by modulating R unique nxn permuted code sets, c (1) (n) to c (R) (n) by successive scalar elements of M(G) .
• the step of using the modulation matrix M(G) comprises a step of operating a scalar times matrix multiplier.
• Fig. la depicts a conventional timing diagram of various Walsh codes used at rates Rc/n and Rc/2n;
• Fig. lb is a simple block diagram depicting a recursive function generator, and which is useful in explaining a prior art Walsh code set generation technique;
• (8) indicates the jth column of w(8) , by including the elements of rows 1:8 and column j ;
• Fig. 4a depicts the details of a jth scalar modulator working on the sequence of permuted matrices for cell #1;
• Fig. 4b illustrates a procedure for generating the codes for cell #1 by modulation the sequence of permuted cell #1 n x n code sets with the G rows of the modulation matrix M(G) ;
• Fig. 5 is a diagram showing code sequences for use in cell #1 for a case described by a first example (Example 1) ;
• Fig. 6a depicts an Equation 10 useful in explaining a matrix used to modulate a sequence of permuted matrices for cell #1, in accordance with a second example (Example 2) ;
• Fig. 6b is a diagram showing code sequences for use in cell #1 for a case described by the second example, where the boxes delineate the intended symbol boundaries at various data rates;
• Fig. 8 is a block diagram of a PN code generation circuit
• Fig. 9 is a simplified block diagram of a synchronous, spread spectrum CDMA fixed wireless communications system in accordance with an embodiment of this invention.
• Fig. 10 is an exemplary frequency allocation diagram of the system of Fig. 9.
• Fig. 11a illustrates an exemplary Hadamard (H) matrix
• Fig. lib illustrates a Reordering Code (RC)
• Fig. lie illustrates a Reordered Hadamard (RH) code matrix in accordance with the invention described in the above- referenced commonly assigned U.S. Patent Application S.N. 09/328,546, filed 6/9/99, entitled "PN Code Selection for Synchronous CDMA” , by Leon Nieczyporowicz , Thomas Giallorenzi and Steven B. Perkins;
• Fig. 12 illustrates an exemplary 8x8 Walsh code matrix, an exemplary reordering code, and the resultant reordered Walsh code matrix, in accordance with the invention described in the above-referenced commonly assigned U.S. Patent Application S.N. 09/328,546, filed 6/9/99, entitled "PN Code Selection for Synchronous CDMA";
• Fig. 13 illustrates an exemplary inversion pattern for application to the reordered Walsh code matrix of Fig. 12, and the resultant inverted, reordered Walsh code matrix, in accordance with that invention
• Fig. 14 is a simplified block diagram of a reordering pattern or code generator and a shift register for reordering a PN code.
• the standard Walsh codes are reordered using a pseudo-random reordering pattern.
• a code set w(n) having elements w ;j (n)
• permutes the columns of w(n) in a random-like fashion to obtain a new code set matrix.
• Additional steps of permuting rows and inverting rows can also be employed to provide a code matrix which has the additional, appealing properties of having a reasonable peak-to-average power ratio when transmitting correlated data on each of the CDMA channels.
• the reordered code matrix is referred to as c ( ) (n) to denote the k th reordering pattern of w(n) , where it is assumed that the rows of w(n) were permuted and possibly inverted and the columns were permuted to obtain c (k) (n) .
• the operations described do not change the fact that the rows of the matrix c ⁇ k) (n) are perfectly balanced. This implies that one of the rows will always be made up of all +1 values (or -1 values if it is inverted) since reordering the all +1 vector of w(n) does not change it.
• the all ones PN code of the set, c (k) (n), may be discarded, leaving n-1 PN codes which are perfectly balanced, mutually orthogonal, and which possess good spectral properties, if the k th reordering pattern is a good one.
• This 19-cell code reuse pattern insures that no two cells are using the same code set, within two cells of one another, in a cellular grid.
• PN codes for multirate synchronous and quasi- synchronous CDMA systems which have good spectral properties, are balanced, and that permit multicell operation.
• This method is shown to be a non-recursive extension of the codes disclosed in the above-referenced commonly assigned U.S. Patent Application S.N. 09/328,546, filed 6/9/99.
• the method of reordering or permuting the columns of a Walsh matrix are extended by using a reordered matrix to modulate the reordered matrices described in the above-referenced commonly assigned U.S. Patent Application.
• c (k) (n) was defined to be the kth code matrix created from w(n) by permuting the rows and columns according to the k th permutation patterns, and by inverting some of the rows of the resulting matrix.
• c ⁇ ) jj (n) was defined to be the j th ⁇ 1 valued chip in the i th row of the code matrix, c (k (n) .
• the code set construction method in accordance with the teachings of this invention may be considered to be based on a round-robin cycling of the code sets assigned to any particular cell or sector.
• FIG. 2 illustrates how, in the first cell, code sets 1, 2 and 3 are used in a round-robin fashion, while in cell 2, code sets 4, 5 and 6 are used in the same way. Since the cross-correlation properties of the various reordered sets are good by design, the relative phase offset of the symbols of one base station and another are irrelevant, and the base station-to-base station synchronization required in the prior art technique disclosed in U.S. Patent No.: 5,751,761 is made unnecessary.
• R sL the lowest desired symbol rate to be used by the system
• G 2 9 .
• M(G) be a GxG modulation matrix whose elements are ⁇ 1 values, obtained by a constrained permutation of the columns of a Walsh matrix of the same size.
• Fig. 3 illustrates a beginning point with a Walsh matrix of size G, and the permutation of the Walsh matrix to form M(G) under the constraints that only certain permutations are allowed.
• the first set of allowable permutations is to permute adjacent columns within pairs (meaning that columns 1 and 2 , 3 and 4 , 5 and 6, 7 and 8 may be permuted with each other, but 2 and 3, 4 and 5, 6 and 7 may not be permuted, for example) .
• the next set of permissible permutations is to permute adjacent pairs of columns within groups of four columns (referred to as quads) .
• columns 1 and 2 may be exchanged with columns 3 and 4 , but not with 5 and 6 or 7 and 8, for example.
• groups of four columns may be swapped with adjacent groups of four columns. For example, columns 1, 2, 3 and 4 may be swapped with columns 5, 6, 7 and 8 as a group. If G> 8, then this process may be extended until the final stage, where the first G/2 columns of w(G) may or may not be permuted with the second G/2 columns of w(G) .
• the permutations can be done in any order, such as columns within pairs, then column pairs within quads, and then column quads within column octals, or, by example, column pairs within quads, followed by columns within pairs, followed by column quads within octals, etc. This process can be extended to larger matrices, such as those having 16, or 32, or 64 columns. Any number of permutations can be done, from zero to every possible permutation within the above-defined constraints.
• the modulation matrix M(G) is used to create the set of available PN codes for the first cell by modulating the R unique nxn permuted code sets, c ⁇ 1> (n) to c (R) (n) stored in shift register 2, with the various elements of M(G) stored in shift register 3, using a scalar times matrix multiplier 4 found in a scalar modulator 1.
• Figs. 4a and 4b it can be seen that the sequence of nxn permuted code sets which were allocated to cell #1 are modulated by the G rows of M(G) .
• M(G) is a GxG matrix
• the modulation sequence repeats once every G clocks, and since the sequence of input matrices repeats every R matrices (and there is one clock transition per matrix input) , it follows that this forms an overall sequence which repeats once every RG matrices, or once every RGn chips.
• the i th modulated matrix sequence output will be
• D (1> . is an n x RGn matrix containing one cycle of the n PN codes corresponding to the i th modulating sequence of M(G) .
• D (1) is a Gn x RGn matrix having elements
• the permuted modulation matrix is used to create D (1) for cell #1, which in this case is a 2n x 6n matrix.
• n (n) , c (2 n (n) and c (3) n (n) are 1 x n vectors with all +1 elements
• the n th row of D ⁇ 1) namely D ⁇ 1> n is a 1 x 6n vector having all +1 values and thus should not be used because of its poor balance and poor spectral properties.
• D ⁇ 1) 2n is a 1 x 6n vector having three groups of n "-1" values followed by n "+1" values respectively. This vector also has poor spectral properties and thus should not be used.
• the remaining 2n-2 rows of D (1) are mutually orthogonal, have perfect balance and have very good spectral properties.
• the first line illustrates the fact that if the codes are to be used at a rate of Rc/n, then there are n-1 codes available (assuming the code based on c ⁇ k) n (n) is eliminated) , namely the ones modulated by M 1 (4) .
• n-1 codes available (assuming the code based on c ⁇ k) n (n) is eliminated)
• M 1 (4) the ones modulated by M 1 (4) .
• M 2 (4) there are 2n-2 codes available, namely those based on M,(4) and M 2 (4) .
• the two codes based on c ck> n (n) are eliminated as in the Example 1.
• Rc/4n there are 4n-4 codes available. At this rate, the codes based on every row of M(4) are utilized.
• the system illustrated in this Example 2 is capable of supporting Rc(n-l)/n symbols per second, independent of the rate of the users. This may equate to n-1 users at Rc/n, 4n-4 users at Rc/4n, or some mix of the rates in between.
• Fig. 8 illustrates one suitable embodiment of the PN code generator 20 for a subscriber unit.
• Fig. 8 shows that the PN code generator 20 includes two memories 22 and 24 implemented in one or two ROMs or, more typically, non-volatile random access memories (RAMs) so that the code matrices may be changed in the field, if necessary.
• the M(G) matrix is stored in the G x G bit "M” memory 24.
• the actual code to be used may then be commanded by a microprocessor 26 via an Address Logic block 28, i.e., which row of the "C” memory 22 to read and which row of the "M” memory to read 24.
• the Address Logic block 28 then uses two internal counters, which are clocked by a chip clock 30 and which are reset by a frame/burst clock 32, to insure that the PN codes of all users have the correct phase.
• the Address Logic block 28 addresses successive chip locations across the Rn-bit row of the "C” memory 22 which was previously commanded by the microprocessor 26.
• n-bit chip counter Every time an n-bit chip counter rolls over, it clocks a G- bit counter which is used to select the current bit of the appropriate row of the "M" memory 24. Thus the "M" memory 24 outputs the same bit for n successive chips and then iterates.
• the entire state machine rolls over every RGn chip clock periods, which represents the repeat length of the synthesized code. Since the length of the frame or burst of a particular system may not correspond to an integer number of RGn chip-periods, it follows that the state machine will be reset to the zero phase every frame or burst.
• Fig. 9 illustrates a Fixed Wireless System (FWS) 10 that is suitable for practicing this invention.
• the FWS 10 employs direct sequence spread spectrum based CDMA techniques over an air link to provide local access to subscribers, and offers very high quality, highly reliable service.
• the FWS 10 is a synchronous CDMA (S-CDMA) communications system wherein forward link (FL) transmissions from a base station, referred to also as a radio base unit (RBU) 12, for a plurality of transceiver units, referred to herein as user or subscriber units (SUs) 14, are symbol and chip aligned in time, and wherein the SUs 14 operate to receive the FL transmissions and to synchronize to one of the transmissions.
• S-CDMA synchronous CDMA
• Each SU 14 also transmits a signal on a reverse link (RL) to RBU 12 in order to synchronize the timing of its transmissions to the RBU 12, and to generally perform bidirectional communications.
• the FWS 10 is suitable for use in implementing a telecommunications system that conveys multirate voice and/or data between the RBU 12 and the SUs 14. As was made evident above, it is not necessary that the RBUs 12 be synchronized to one another, when employing the orthogonal PN code sets in accordance with this invention.
• the RBU 12 includes circuitry for generating a plurality of user signals (USER_1 to USER_n) , which are not shown in Fig. 1, and a synchronous side channel (SIDE_CHAN) signal that is continuously transmitted. Each of these signals is assigned a respective PN spreading code and is modulated therewith before being applied to a transmitter 12a having an antenna 12b. When transmitted on the FL the transmissions are modulated in phase quadrature, and the SUs 14 are assumed to include suitable phase demodulators for deriving in-phase (I) and quadrature (Q) components therefrom.
• the RBU 12 is capable of transmitting a plurality of frequency channels.
• each frequency channel includes up to 128 code channels, and has a center frequency in the range of 2 GHz to 3 GHz.
• the RBU 12 also includes a receiver 12c having an output coupled to a side channel receiver 12d.
• the side channel receiver 12d receives as inputs the spread signal from the receiver 12c, a scale factor signal, and a side channel despread pn code. These latter two signals are sourced from a RBU processor or controller 12e.
• the scale factor signal can be fixed, or can be made adaptive as a function of the number of SUs 14 that are transmitting on the reverse channel.
• the side channel receiver 12d outputs a detect/not detect signal to the RBU controller 12e for indicating a detection of a transmission from one of the SUs 14 , and also outputs a power estimate value ⁇ .
• a read/write memory (MEM) 12f is bidirectionally coupled to the RBU controller 12e for storing system parameters and other information, such as SU timing phase information and power estimate values.
• a Network Interface Unit (NIU) 13 connects the RBU 12 to the public network, such as the public switched telephone network (PSTN) 13a, through analog or digital trunks that are suitable for use with the local public network.
• the RBU 12 connects to the NIU 13 using El trunks and to its master antenna 12b using a coaxial cable.
• the SU 14 communicates with the RBU 12 via the radio interface, as described above.
• the SU-RBU air link provides a separate 2.72 MHz (3.5 Hz including guardbands) channel in each direction separated by either 91MHz or 119 MHz of bandwidth.
• the nominal spectrum of operation is 2.1-2.3 GHz or 2.5-2.7 GHz.
• the system is designed such that the frequency can be varied from 1.8 to 5 GHz provided the spectral mask and separation between transmit and receive frequencies is maintained as per ITU 283.5 specification.
• the RBU 12 may transmit in the 3 ' frequency band and receive in the 3 frequency band, and the SU 14 transmits in the 3 frequency band and receives in the 3' frequency band.
• one column is all ones.
• some correlated data may occur (e.g., a synchronization pattern, a particular silence pattern from a voice encoder, etc.) .
• some of the rows of the Walsh matrix may be inverted. This prevents the all ones column from resulting in a large correlation peak in the composite signal, which may cause a problem in the presence of nonlinear impairments (i.e. clipping) .
• the codes have different auto-correlation and cross-correlation properties.
• an asynchronous channel e.g. , the side channel
• scrambling the Walsh codeset is typically accomplished by generating another PN code (such as the above-referenced cover code) of the same length as the Walsh code, or a larger length, and then XOR'ing each code in the Walsh set with the cover code.
• PN code such as the above-referenced cover code
• reordering the Walsh codeset is accomplished by exchanging columns of the Walsh codeset matrix, and also possibly inverting one or more of the codewords in the codeword set to avoid degradation due to correlated data.
• the balanced,properties of the Walsh codeset are maintained, and the number of +l's is equal to the number of -l's (or O's) in each codeword (except for the all ones codeword) .
• the number of +l's is equal to the number of -l's (or O's) in each codeword (except for the all ones codeword) .
• Fig. 11a illustrates an exemplary Hadamard matrix (treating a -1 as a 0) .
• Reordered Hadamard codes are constructed by reordering the columns of the Hadamard matrix.
• the Hadamard matrix (H) of Fig. 11a is reordered using the Reordering Code (RC) shown in Fig. lib, and the resulting Reordered Hadamard (RH) code matrix is shown in Fig. lie. Note that the third column has been moved to the first column position, and columns 1 and 2 have been shifted to the right by one column position.
• RC Reordering Code
• Fig. 14 shows a block diagram of a random number generator 16 that outputs a Reordering Pattern or Code 16a to a shift register 18 having feedback through an XOR function 20.
• the reordering code can be generated using any random or pseudorandom sequence generator as shown in Fig. 14. For example, a random sequence from 1 to N (where N is the length of the Walsh code, or less) is generated. Then each of the columns of the Walsh code is reordered according to its location in the reordering pattern or code sequence, as shown in Fig. 12.
• the resultant codeset cannot be achieved by simply applying a cover code to the original Walsh codeset, since the only way to achieve the all ones codeword is to use one of the codes of the Walsh set as the cover code, and using one code of the Walsh set as a cover code simply renumbers the codewords.
• inverting codewords An important goal when inverting codewords is to provide a simple means to reduce the peak signal level when transmitting correlated data.
• invert codewords one first defines an inversion pattern. Then the inversion pattern is applied by multiplying each element in a row by its corresponding element in the inversion pattern. Thus row 1 in the reordered codeset is multiplied by row 1 in the inversion pattern, etc.
• the resulting waveform is the sum of each column of the codeset.
• the codeset For the reordered codeset it is assumed that all users are transmitting a 1 for the data and, therefore, one can sum each column to determine that the transmitted waveform is:
• tx_waveform_reordered_code [0 0 0 0 8 0 0 0].
• tx_waveform_reordered_code_w_inversion [22 -622222]. Note that while no attempt was made to optimize the example inversion code, the peak of the transmitted signal is reduced from 8 to 6 (only the magnitude is of interest) . While this case appears to give but a slight improvement, when operating with codesets of size 128 the peak can be reduced from 128 to approximately 75. This beneficially allows operation with correlated data without clipping.
• the RBU 12 of Fig. 9, or some other processor has the ability to generate the reordered (and possibly inverted) codes as shown in Figs, lla-llc, 12 and 13, and to then also modulate or multiply same by the permuted matrix M(G) as described in detail above, in order to generate the orthogonal PN codesets for use in the multirate FWL CDMA system 10.

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